It doesn't need to be the malicious. Let's assume the mutation happens when the person is in their 20s, which would be ideal. If this occurred any time in hist

It doesn't need to be the malicious. Let's assume the mutation happens when the person is in their 20s, which would be ideal. If this occurred any time in history up to the 1800s, would you not still be likely to die of disease, cancer, or other injury? Life expectancy hasn't increased because we are better at slowing aging, it has increased because we are better at not dying of the things that kill us. If the mutation happened invitro, did the child survive birth? If it did survive but never grew, it wouldn't reproduce, and may have been abandoned if it were too much of a burden. There are many scenarios where this mutation could have occurred but was never passed on.